The present invention relates to a microwave diode balanced mixer circuit. More particularly, it relates to such a circuit of the type which embodies the techniques sometimes called image and sum enhancement, which are employed in order to minimize conversion loss. It also relates to such a circuit of the type which utilizes a coplanar strip transmission line-slot transmission line hybrid junction for balanced semiconductor diode modulation. The present invention is of special utility in operation at frequencies as high as the K band or higher.
Well known are the image and sum enhancement techniques for control of the impedance at each of the mixer terminals and of each of the frequencies of importance. The frequencies of importance are the modulation products which exist according to the heterodyne principle by which the mixer operates. The received signal at frequency.omega..sub.S, together with a higher level signal from a local oscillator (LO), is applied to a mixer diode to derive an intermediate frequency, equal to the difference between the received signal frequency and the LO frequency. Harmonics of the LO frequency (.omega..sub.LO) are also important. However, as the harmonic amplitude falls off approximately as 1/n.sup.2 (where n is the harmonic number), the power in the third harmonic and higher is quite small. Thus, only the effects of the fundamental and second harmonic are significant. The signal is mixed with .omega..sub.LO and 2.omega..sub.LO, producing the sum frequency .omega..sub.LO + .omega..sub.S, the difference (or intermediate) frequency.omega..sub.LO - .omega..sub.S, and the image frequency 2.omega..sub.LO = .omega. .sub.S.
Prior art integrated circuit forms of image and sum enhanced mixers are known and described in publications. For example, see H. A. Watson, "Microwave Semiconductor Devices And Their Circuit Application", New York, McGraw-Hill, 1969; and J. B. Cahalan et al, "An Integrated, X-Band, Image And Sum Frequency Enhanced Mixer With 1 GHz IF", 1971 G-MTT Symposium Digest, pp. 16-17. However, prior to the present invention, the only such circuits which have achieved general acceptance have been single ended (unbalanced) mixers as opposed to balanced mixers. These are inherently subject to limitation of narrow band operation imposed by use of a narrow band filter for image termination control.
The publication, L. E. Dickens, "Low Conversion Loss Millimeter Wave Mixers", 1973 G-MTT Symposium Digest, June 1973, pp. 66-68, describes and explains design considerations for operation of microwave mixer diode for lowest conversion loss. Use of these considerations in conjunction with those described and explained in M. R. Barber, "Noise Figure And Conversion Loss Of The Schottky Barrier Mixer Diode", IEEE Transactions on MTT, Vol. MTT-15, No. 11, pp. 629-635, November 1967, enables a theoretical calculation of comparative minimum mixer conversion losses for the image and sum enhanced mixer and the broadband mixer, respectively. It can be shown that such theoretically calculated minimum conversion losses for the case of a signal frequency center at 9.3 GHz using gallium arsenide (GaAs) Schottky barrier diodes for high frequency operation are about 1.0dB for the image and sum enhanced mixer and about 3.5dB for the broadband mixer. Thus, there is about a 2.5dB theoretically calculated advantage which potentially may be gained by image and sum enhancement techniques with balanced diode mixers.
Balanced semiconductor diode modulation utilizing the hybrid junction formed by the intersection of a slot transmission line with a coplanar strip transmission line is known. An example of such utilization is disclosed in U.S. Pat. No. 3,678,395, Hunton et al. Other prior art references disclosing use of slot-type distributed transmission line characteristics in providing balanced mixer circuit operation which are worthy of note are: U.S. Pat. No. 3,735,267 Napoli; U.S. Pat. No. 3,772,599 Ernst and Yuan.
However, none of these employ image and sum enhancement techniques.
There is also a continuing need for improvements in integrated circuit balance mixer circuits to provide better electrical performance, improved reproducibility and lower production cost.